1. Field of the Invention
The present invention generally relates to automotive rectifier assemblies, and more particularly, to a method and apparatus for reducing the operating temperature of an automotive rectifier assembly. In a specific construction, the present invention relates to an automotive rectifier assembly for converting a polyphase alternating current to direct current by means of silicon semiconductor diodes.
2. Description of Related Art
Automotive alternator design has followed the trend in automotive manufacturing of decreasing manufacturing costs, air pollution and weight. Although alternators have become smaller, the electrical energy output requirements have increased. Compact alternators can often not dissipate heat out of the rectifier bridge fast enough to prevent semiconductor failures. This is particularly true during the summer months when the ambient temperatures are relatively high, providing reduced heat transfer resulting in a higher alternator failure rate, often attributed to semiconductor failure.
Generally, recharging an automobile battery requires a current between 40 and 50 amperes. Combined with the energy requirements of the air conditioning system, computer modules, a car radio, fans, and lighting systems, the overall current consumption can exceed 150 amperes.
Once an alternator is installed in a vehicle, all semiconductor diodes are electrically connected to the battery, completing a number of potential short circuit paths to the ground. The wiring harness of the charging system usually incorporates a 12 AWG fuse link safety circuit, for fire and meltdown protection.
Heat and voltage transients degenerate semiconductor switches and cause undesired reverse current leakage through the semiconductor junction. The leakage can lead to excessive junction heating. Once overheated, the semiconductor switch may be damaged beyond recovery. The semiconductor switch can also lose its blocking characteristics and allow current to flow in both directions. The excessive heat can then cascade into and damage other semiconductor switches of the same circuit.
Generally, there are no cut out relays or switches that open the semiconductor circuits of the rectifier system when a vehicle is shut down. Therefore, the circuits usually remain electrically “HOT” when the vehicle is shut down. Further, the alternator cooling system is also shut down when a vehicle is not operating, thus leaving the circuits thermally vulnerable. Latent heat remains in the thick rectifier housing and conducts back into the semiconductors. Thus, the alternator of the unattended shut-down vehicle is slowly heating up, as heat cascades from one semiconductor to another, causing semiconductor failures, and generating enough heat so as to potentially ignite an under-the-hood fire.
When the semiconductors fail, the current level is generally not high enough to melt the 12 AWG fuse. The semiconductors usually fail with a combined resistance of approximately 0.3 ohm. Thus, a 40 ampere current flows through the failed circuit. The level of current translates to 480 watts generated within the rectifier case. The 480-watt power output is approximately 13 times greater than an average 37 watt soldering iron used in the electronics industry.
The failed semiconductors become high wattage heaters (controlled by the resistance of the hot silicon), overheating the path through the copper components, melting the plastic affixing the terminals, melting the epoxy fillers, and igniting any grease or oil on the wiring harness insulation. Furthermore, the leakage path does not conduct enough current to melt the 12 AWG fuse link. Therefore, there is only an appearance of safety when employing the fuse link. Once started, the meltdown continues until the battery is discharged or manually disconnected. Rectifiers that fail without a catastrophic failure are still a nuisance to the general public because of the required service calls, the towing, and the repair costs.
Therefore, there is a need for dissipating heat from a rectifier assembly to reduce semiconductor failure. The need also exists for a rectifier assembly that is manufactured without overstressing the semiconductors. The need further exists for a rectifier assembly can dissipate sufficient heat to reduce system failure, and particularly semiconductor failure, without sacrificing mechanical robustness of the system or increasing the size and cost of the assembly.